Two binarization methods are known in the art. One is a fixed-type binary coding system in which the threshold value is fixed, and the other is a floating type binary coding system in which the threshold value may be varied. The former system suffers from a difficulty that when the video signal is changed by optical disturbance or variation of the reflection factor of an object, the strokes of the resultant binary-coded picture become bolder or finer i.e., the video representation is unstable.
The latter system encounters no trouble when the video signal changes as a whole, because the threshold value is changed in response to the variation of the video signal. However, in the case when the object's contrast changes depending on the positions of scanning lines, it is impossible to obtain a suitable picture merely by regularly varying the threshold value when the video signal changes as described above.
This will be described in more detail.
FIG. 1 is a plan view showing one example of a keyboard. Each of a group of key tops 1a on the left side of FIG. 1 has a black character (digit) against a light background, and each of a group of keytops 1b on the right side of FIG. 1 has a white character (arrow mark) against a dark background.
The key tops on the left side of FIG. 1 are different in contrast from those on the right side. Therefore, it can be readily understood that it is impossible to obtain an excellent binary-coded picture thereof according to a simple floating type binary coding system.